Plastic Shrinkage Behaviour and Mitigation Measures for Fibre Reinforced Concrete

Plastic shrinkage (in fresh concrete) can cause the initiation of early-age cracking, which can eventually evolve into larger cracks, compromising durability and reducing overall service-life. Plastic shrinkage cracks can also facilitate the ingress of harmful chemicals (e.g. chlorides) into the concrete and increase the possibility of corrosion of steel bars. Plastic shrinkage is affected by various parameters, including the properties and proportions of the concrete constituents, as well the environmental conditions (temperature, relative humidity, and wind speed). These parameters affect the bleeding and evaporation rates of surface water, and are thus directly related to the formation of plastic shrinkage cracks. This research aims to investigate the mechanisms of plastic shrinkage and plastic shrinkage cracking, with a view of developing cost-effective and sustainable mitigation strategies to control early-age cracking. The main parameters investigated in this research study are: 1) fibre type (i.e. manufactured steel fibres (MSF) and recycled tyre steel fibre (RTSF) from post-consumer tyres) and fibre content (10, 20, and 30 kg/m3); 2) water to cement ratio (0.5 – 0.6); 3) environmental conditions (wind speed and temperature); and 4) concrete curing methods and use of admixtures. RTSF was found more effective in preventing crack development than MSF, at the same fibre content. When fresh concrete was exposed to environmental conditions typical of Saudi Arabia (high temperatures and wind speed), it was found that while 30 kg/m3 of RTSF can control plastic shrinkage cracks at lower w/c ratios (0.5 and 0.55) and in low and mid environmental conditions (T=28-36 °C, and wind speed=3-4.6 m/s), for higher w/c ratio (0.6) and in more extreme conditions (T=45 °C and wind speed=7 m/s), the use of 40 kg/m3 of RTSF fibre was required to completely eliminate surface plastic shrinkage cracks. All of the different mitigation strategies commonly used in construction were successful in restraining plastic shrinkage cracks, albeit with different associated cost and efficiency. The use of cold water (7 °C) in the mix, showed the most benefits in terms of cost, quality, and time, while the use of RTSF gave optimal results in terms of performance and overall structural benefits. Based on the main results of this study, recommendations are made to control plastic shrinkage in pavements. The recommendations consider not only the overall mechanical performance of the concrete (crack initial time, crack reduction ratio) and construction method, but also cost, speed of construction/application, and sustainable.